1. Field of the Invention
This invention relates to an optical head for recording and/or reproducing the information for an optical disc, which permits optical information recording and/or reproduction, such as magneto-optical disc or phase-change optical disc, and a recording and/or reproducing apparatus, employing this optical head.
This application claims the priority of the Japanese Patent Applications No. 2003-155675 filed on May 30, 2003, and No. 2004-120747 filed on Apr. 15, 2004, the entirety of which is incorporated by reference herein.
2. Description of Related Art
There has so far been known a recording and/or reproducing apparatus including a light source and an optical system which, for reproducing optical discs having different formats, such as CD (Compact Disc) or DVD (Digital Versatile Disc), is capable of radiating laser light beams of different wavelengths for coping with the respective formats.
Referring to FIG. 45, an optical system 201, provided to this sort of the recording and/or reproducing apparatus, includes, in an arraying order corresponding to the ongoing direction of the optical path, a double wavelength light source 211, selectively radiating laser light beams of respective different wavelengths to an optical disc 204, a diffractive lattice for three beams 212, for splitting the outgoing light radiated from the double wavelength light source 211 into three beams, a beam splitter 213 for separating the outgoing light and the return light from the optical disc 204 from each other, an aperture stop 214 for restricting the outgoing light to a preset numerical aperture NA, a double wavelength objective lens 215 for converging the outgoing light to the optical disc 204, and a light receiving unit 216 for receiving the return light from the optical disc 204.
As the double wavelength light source 211, a semiconductor laser is used, and selectively radiates a laser light beam of, for example, approximately 780 nm, and another laser light beam of approximately 650 nm, from a light emitting point 211a. 
For producing tracking error signal by the so-called three-beam method, the diffractive lattice for three beams 212 splits the outgoing light, radiated from the double wavelength light source 211, into three beams, namely an order zero light beam and±order one light beams.
The beam splitter 213 includes a half-mirror surface 213a for reflecting the outgoing light, radiated from the double wavelength light source 211, in the direction towards the optical disc 204. The beam splitter reflects the outgoing light, radiated from the double wavelength light source 211, in the direction towards the optical disc 204, while transmitting the return light from the optical disc 204 onto the light receiving unit 216 to separate the optical path of the outgoing light beam from that of the return light.
The light receiving unit 216 includes, on a light receiving surface 216a, a photodetector for a main beam 217, as later explained, for receiving the order zero light beam split from the return light by the diffractive lattice for three beams 212, and a set of photodetectors for side beams, not shown, for receiving the±order one light beams, split from the return light by the diffractive lattice for three beams 212.
In the optical system 201, an astigmatic method is used for detecting focusing error signals. Thus, as shown in FIGS. 46(a) to 46(c), a light receiving surface of a photodetector for a main beam 217, receiving the return light, is substantially square-shaped, and is split into four equal light receiving areas A to D by a pair of mutually orthogonal splitting lines passing through the center of the light receiving surface. A pair of photodetectors for side beams is arranged on both sides of the photodetector for a main beam 217.
Referring to FIG. 45 the optical components of the optical system 201 are arranged on an ongoing path optical from the double wavelength light source 211 to the optical disc 204 so that image points as conjugate points of light emitting points 211a, 211b of the double wavelength light source 211 as object points are disposed on a recording surface 205 of the optical disc 204.
The optical components of the optical system 201 are also arranged on a return path from the optical disc 204 to the light receiving unit 216 so that, with the point on the recording surface 205 of the optical disc 204 as object point, the image points as conjugate points are located on the light receiving surface of the photodetector for the main beam 217 of the light receiving unit 216.
Hence, the light emitting points of the double wavelength light source 211 of the optical system 201 are in a conjugate relationship with respect to the points on the light receiving surface of the photodetector for the main beam 217 of the light receiving unit 216.
The method for producing a focusing error signal by the light receiving areas A to D of the photodetector for the main beam 217 is hereinafter explained.
First, in case the double wavelength objective lens 215 is at an optimum position relative to the recording surface 205 of the optical disc 204 and is focused with respect to the recording surface 205 of the optical disc 204, that is, in a just-focus state, the profile of a beam spot on the light receiving surface of the photodetector for the main beam 217 is circular, as shown in FIG. 46(b).
However, when the double wavelength objective lens 215 has excessively approached to the recording surface 205 of the optical disc 204, the double wavelength objective lens deviates from the just focus state, such that, due to the astigmatism generated as a result of the passage through the composite optical component 212 of the return light separated by a diffraction lattice for a beam splitter 212b, the beam spot on the light receiving surface of the photodetector for the main beam 217 is of an elliptical profile with the long axis of the ellipsis astride the light receiving area A and the light receiving area C, as shown in FIG. 46(a).
If the double wavelength objective lens 215 is moved excessively away from the recording surface 205 of the optical disc 204, the double wavelength objective lens deviates from the just focus state, such that, due to the astigmatism generated as a result of the passage through the composite optical component 213 of the return light separated by the diffraction lattice for a beam splitter 212, the beam spot on the light receiving surface of the photodetector for a main beam 217 is of an elliptical profile with the long axis of the ellipsis astride the light receiving area B and the light receiving area D, as shown in FIG. 46(c). The beam spot profile in this case is elliptical with the long axis direction inclined 90° from the beam spot profile shown in FIG. 46(a).
With return light outputs SA, SB, SC and SD from the respective light receiving areas A to D of the photodetector for the main beam 217, the focusing error signals FE may be calculated as shown by the following equation (6):FE=(SA+SC)−(SB+SD)  (6).
That is, in the just-focus state of the photodetector for the main beam 217, in which the double wavelength objective lens 215 is at the focused position, the focusing error signal FE, calculated by the above equation (6), is zero, as shown in FIG. 46(b).
If, with the photodetector for the main beam 217, the double wavelength objective lens 215 has excessively approached to or moved excessively away from the recording surface 205 of the optical disc 204, the focusing error signal FE is positive or negative, respectively.
The tracking error signal TE may be produced by the photodetectors for side beams receiving the±order one light beams, split by the diffraction lattice for three beams 212 and calculating the difference of the respective outputs of the photodetectors for side beams.
With the optical pickup device, having the optical system 201, constructed as described above, the double wavelength objective lens 215 is actuated and displaced, based on the focusing error signal FE obtained by the photodetector for the main beam 217 of the light receiving unit 216, and the tracking error signal TE obtained by the photodetector for side beams, whereby the double wavelength objective lens 215 is moved to the focused position with respect to the recording surface 205 of the optical disc 204, such that the outgoing light is focused on the recording surface 205 of the optical disc 204 to reproduce the information from the optical disc 204.
However, in the above-described optical system, in which the beam splitting is made on the photodetector, the requirement for position accuracy on the light receiving surface of the photodetector is extremely severe. Additionally, since the focusing error signals are produced thanks to the severe position accuracy of the light emitting unit, light receiving unit or other components, an extremely severe tolerance is imposed on the shape or manufacture methods of base components of the optical pickup, or on the shape or the arranging method of other components.
For example, in an optical system, shown in FIG. 45, the optical axis of the return light is deviated by an error in the mounting angle or the error in thickness of the beam splitter 213. If the optical axis of the return light is deviated to the slightest extent in one or the other direction from the center of the photodetector for the main beam 217, the output for the just-focus state is not zero, and hence the focusing error FE is offset.